OS 10.5 - want to show log in screen after screen saver

Is there a way I can get Leopard to show me the log in screen after the computer goes to sleep or the screen saver has been on for a certain amount of time. If I walk away from my computer for say 15 min and my screen saver comes on after 10 min then my wife jump onto the computer after it comes out of screen saver I want it to show the log in screen instead of going back into my account. How can I do this?